其他的麻烦，写写SQL~
不一定对哦~~自己验证吧~
4-1
DELETE FROM book
WHERE date BETWEEN '1998-01-01' AND '1998-12-31'
4-2
SELECT SUM(amount)
FROM book
WHERE date BETWEEN '1998-01-01' AND '1998-12-31'
4-3
SELECT COUNT(1)
FROM book
WHERE date BETWEEN '1998-01-01' AND '1998-12-31'
5-1
SELECT A.Name, B.grade
FROM Table1 A LEFT JOIN Table2 B ON (A.Id = B.Id)
WHERE A.age = '22'
AND B.Course = '语文'
5-2
SELECT Course, AVG(grade)
FROM Table2
GROUP BY Course
